As a semiconductor device in which pixels each provided with a photosensor are arranged in a matrix, an image sensor is known. The image sensors are provided in many portable devices such as a digital camera or a cellular phone as imaging elements. In recent years, the definition of imaging has been increased, the portable devices have been downsized, and power consumption has been reduced; therefore, a pixel in the image sensor has been made smaller.
As an image sensor in general use, two kinds of sensors are known: a charge coupled device (CCD) sensor and a complementary metal oxide semiconductor (CMOS) sensor. The CCD sensor is an image sensor in which charge is transmitted by a vertical CCD and a horizontal CCD. The CMOS sensor is an image sensor formed using a CMOS process. In the CMOS sensor, reading of charge can be controlled per pixel unit by switching of a MOS transistor.
The CCD sensor has high sensitivity; however, when excessive light is incident on part of a photodiode, charge which is greater than or equal to the maximum permissible value flows into a vertically transfer CCD, and a longitudinal emission line, called a smear, is generated. Further, the CCD has problems such as high production cost with a dedicated process and large power consumption due to many power sources.
In contrast, although the CMOS sensor has lower sensitivity than the CCD sensor, a general-purpose CMOS process can be utilized and circuits can be integrated in one chip. Therefore, the CMOS sensor can achieve low cost and low power consumption. Further, the CMOS sensor amplifies a signal in a pixel and outputs it; therefore, the influence of noise can be reduced. In addition, because the method for transmitting charge by the CMOS sensor is different from that by the CCD sensor, a smear is not generated.
However, for the conventional CMOS sensor, a rolling shutter system by which pixels arranged in a matrix are driven per row has been adopted. This rolling shutter system had a problem that an image was warped when an image of an object which moves fast was taken. In contrast, for the CCD sensor, a global shutter system by which charge is accumulated at the same time in all the pixels is adopted.
As a means in which the CMOS sensor with global shutter is used, Patent Document 1 discloses a technique in which a mechanical shutter is used together to control the current of a photodiode. In addition, Patent Document 2 discloses a technique in which a path ejecting unwanted charge which is generated in a photodiode after light exposure is terminated is provided to suppress leakage of accumulated charge.